The present invention relates to light-emitting polymers. More specifically, the present invention relates to light-emitting polymers in which the light-emitting efficiency is improved, and the color tuning is superior, the structure being such that the main chain is composed of poly(p-phenylenevinylene), and the side chain is a phenyl silicon with aliphatic alkyl groups.
Since semiconductor devices have been developed, the electronic technology has achieved an astounding progress in improving the cultural living of mankind. In particular, optical communications and multi-media have rapidly progressed in recent times to accelerate progress toward the present information society. Accordingly, optoelectronic devices which utilize the conversion of photons to electrons and vice versa has become the core of the modem electronic industries.
Optoelectronic devices are roughly classified as light emitting devices, light receiving devices, and combined devices. So far, most displays have been the light receiving type, whereas the electroluminescent display (ELD), a light emitting type, is fast in response speed, requires no backlights, and has superior brightness and other advantages. Therefore, the ELD is being briskly studied in recent times because of its applicability to future color display devices.
This electroluminescence phenomenon has been introduced to practical use by utilizing GaN, ZnS, SiC and the like in inorganic semiconductor devices. However, in the inorganic ELD, a driving voltage of AC 200 V or more is required, and the ELD is manufactured by applying a vacuum deposition process. Therefore, it cannot be applied to a large-scale device, and the manufacturing cost is also high.
Besides the inorganic ELD, organic and polymer ELDs had been developed. Pope et al reported the EL phenomenon of an organic material in 1963, and Tang et al. of the Eastman Kodak company disclosed in 1987 a light emitting device which is manufactured with xcfx80-conjugated alumina-quinone (Alq3). In the device, the quantum efficiency is 1% at less than 10 V, and the brightness is 1000 cd/m2. Since the disclosure of the device, many studies have been carried out. The synthesizing process is simple, and therefore diversified polymeric materials can be synthesized, and color tuning can also be achieved. However, the polymeric materials of the device are poor in processability and thermal stability, and when the voltage is applied to the device, rearrangement of the polymer molecules occurs due to the joule heat that is generated in the light emitting layer. The rearrangement of the molecules causes critical damage to the light emitting efficiency and life expectancy of the device. In order to compensate for this shortcoming, there has been developed a polymeric material having a particular structure for the organic ELD, the so-called xe2x80x9cxcfx80-conjugated polymerxe2x80x9d. In the polymer, due to the overlapping of the xcfx80-electron wave functions of the main chain of the polymer, the energy level is divided into two bands, and the semiconductor properties of the polymer depend on the energy difference between the two energy bands. Through adjustments of the band gap between the two energy bands, a full color can be realized.
In 1990, researchers at Cambridge University in England first disclosed an ELD which utilizes poly(p-phenylenevinylene) (PPV) as a polymer having conjugated double bonds. Since then, ELDs using conjugated polymers have been widely studied. There has recently been developed an ELD using a particular polymer, which surpasses the inorganic semiconductor ELD in efficiency in the visible light region. Further, light emitting polymers having a red, green or blue color have been developed so that full coloring would be achievable. However, if full coloring is to be realized, other problems such as light emitting efficiency, driving voltage and the like, must be solved.
In case of a xcfx80-electron conjugated polymer derivative, i.e., poly(p-phenylenevinylene) (PPV), which is used as a polymeric material for the typical organic ELD, obstacles to mass production of the polymer include factors such as insufficiency of repeatability of the polymer synthesis and device, purification of the polymer, solubility in organic solvents, polymerization period, and manufacturing process. Further, in order to overcome the joule heat when driving the device, the Tg and molecular weight of the polymer should be very high.
In general, the PPV organic EL material of the prior art has the following disadvantages. First, the polysulfonium which is a precursor of PPV requires too long a polymerization time, is prepared in a low yield and is expensive to manufacture. Second, in order to prepare a perfect PPV derivative, the sulfonium has to be completely removed, but this is very difficult. Third, in the case where a thin film having a thickness of 600 xc3x85 is formed, pin holes are formed on the layer when the unreacted sulfonium is removed, thereby reducing the uniformity of the film. Fourth, it is very difficult to synthesize a soluble PPV because the synthesis conditions are very fastidious. Fifth, the PPV manufacturing process is harmful to the environment.
Accordingly, the present inventors have developed light emitting polymers in which the light emitting efficiency is improved, and the color tuning is superior, the structure being such that the main chain is composed of poly(p-phenylenevinylene), and the side chain is a phenyl silicon with aliphatic alkyl groups.
A feature of the present invention is the provision of a new light-emitting polymer with good light emitting efficiency, the structure of the polymer being such that the main chain is composed of poly(p-phenylenevinylene), and the side chain is a phenyl silicon with aliphatic alkyl groups.
Another feature of the present invention is the provision of a light-emitting polymer having an excellent solubility in organic solvents by introducing a phenylsilicon derivative as a side chain.
A further feature of the present invention is the provision of a light emitting polymer having an improved surface property when used in forming electrodes.
A further feature of the present invention is the provision of a light-emitting polymer having improved film formability.
A further feature of the present invention is the provision of a new light emitting polymer with good color tuning, which is prepared by a copolymerization process such that the main chain is composed of poly(p-phenylenevinylene), and the side chain is a phenyl silicon with aliphatic alkyl groups and an MEH-PPV (methoxyethylhexyloxy-PPV).
The above features and other advantages can be attained by the descriptions presented below.
In accordance with one aspect of the present invention, a light emitting polymer is provided that includes a main chain composed of poly(p-phenylenevinylene) (PPV) units and side chains composed of phenyl silicon groups having aliphatic alkyl groups. The polymer is represented by the following formula (I): 
wherein R1, R2 and R3 are selected from the group consisting of linear aliphatic alkyl groups, branched alkyl groups and fluorinated alkyl groups.
In accordance with another aspect of the present invention, a light emitting copolymer is provided which is prepared by copolymerizing (a) a polymer including poly(p-phenylenevinylene) as the main chain and phenyl silicon groups with aliphatic alkyl groups as the side chain and (b) an MEH-PPV (methoxyethylhexyloxy-PPV). The light emitting copolymer is represented by the following formula (II): 
where R1, R2 and R3 are selected from the group consisting of a linear aliphatic alkyl group, a branched alkyl group and a fluorinated alkyl group, x is about 0.1-0.9 and y is about 0.9-0.1.
Electroluminescent devices including the inventive light emitting polymers and copolymers, and methods of producing the inventive copolymers, are also provided.
Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.